Cytochrome oxidase is the terminal complex of the mitochondrial respiratory chain. In Saccharomyces cerevisiae, this enzyme is composed of seven different subunit polypeptides. The three largest subunits are encoded in the mitochondrial genome and are synthesized in the organelle. The regions of mitochondrial DNA containing mutational sites affecting cytochrome oxidase biosynthesis have been sequenced. These studies have provided the complete nucleotide sequences of the genes for subunits 1, 2 and 3 of yeast cytochrome oxidase and indirectly of the amino acid sequences of the proteins. The subunits 2 and 3 are encoded in genes that are co-linear with the protein sequences. Subunit 1, however, is encoded by a mosaic gene with an overall length of 10 kilobases consisting of 8 exons and 7 introns. Studies are currently underway to define the mechanism of transcription and splicing of the mitochondrial cytochrome oxidase messengers.